Light generated or reflected from objects present in the natural world have values of inherent wavelengths. Image sensors use a characteristic of semiconductor devices that are responsive to external light energy to capture an reflected. Each pixel within the image sensor senses the light generated from the object and converts the sensed light into an electrical signal.
Image sensors can be classified into a CCD (charge coupled device), which is based on a silicon semiconductor, and a CMOS (Complementary Metal Oxide Semiconductor) image sensor, which is fabricated using a sub-micron CMOS process.
Among these two types of image sensors, the CCD includes MOS (Metal Oxide Semiconductor) capacitors arranged adjacent to one another in which charge carriers are stored before being transferred. However, the CCD has a complex driving scheme, high power consumption, and a large number of mask processes, which hinders the integration of signal process circuits within the CCD. To overcome obese drawbacks, the CMOS image sensor is actively being researched and developed.
The CMOS image sensor has a photo diode (PD) and a plurality of MOS transistors formed within a unit pixel, and detects signals in a switching scheme to realize or recreate an image. Such CMOS image sensors have the advantages of lower fabrication cost, lower power consumption and easier integration with peripheral circuit chips, as compared to the CCD. More specifically, the CMOS image sensor is fabricated in the CMOS process as described above, and therefore, can be easily integrated with peripheral circuits such as amplifiers and signal processors. Therefore, the CMOS image sensor has a lower fabrication cost than CCD image sensors and has a relatively rapid processing speed. In addition, the CMOS image sensor can reduce power consumption to as low as 1 percent of that of a CCD.
However, conventional CMOS image sensors have a dynamic range disadvantage (e.g., a narrow or limited dynamic range). Dynamic range refers to the ratio between the largest and smallest pixel values in a unit frame. Namely, the conventional CMOS image sensor has difficulties in representing both very dark and very bright areas simultaneously in the unit frame.
To compensate for such a disadvantage of the CMOS image sensor, attempts nave been made to increase the capacitance of a floating diffusion region for every unit pixel to achieve a wide dynamic range.
FIG. 1 is a circuit diagram showing a unit pixel 100 in a CMOS image sensor having a wide dynamic range, in accordance with the related art. A unit pixel of the image sensor includes a photo diode PD, a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, a select transistor Sx, and a capacitance adjusting transistor Mx. The photodiode PD receives light and generates photocharges or photo current. The transfer transistor Tx transfers the photocharges generated by the photos PD to a floating diffusion region FD. The reset transistor Rx discharges the photocharges in the floating diffusion region FD to reset the floating diffusion region FD. The drive transistor Dx, which functions as a source follower buffer amplifier, amplifies and converts the photocharges in the floating diffusion region FD into a voltage signal. The select transistor Sx selects a unit pixel for addressing and/or reading. The capacitance adjusting transistor Mx allows a capacitor Cm to connect to or disconnect from the floating diffusion region FD.
In the unit pixel, the capacitor Cm may be connected to the floating diffusion region FD to increase the capacitance of the unit pixel when the floating diffusion region FD is nearly saturated by the photocharges transferred from the photodiode PD. However, since the capacitance is adjusted with respect to all of the pixels in a unit frame simultaneously, all of the pixels in the unit frame have an increased capacitance, although some of the pixels do not require the increased capacitance.
For example, as shown in the frame in FIG. 2A, first pixel and second pixel 2 are in a non-saturated state (e.g., as indicated by their respective semi-shaded areas) and third pixel 3 and fourth pixel 4 are in saturated state (e.g., as indicated by their respective non-shaded areas). In this case, although it is not necessary for the first and second pixels 1 and 2 to have an increased capacitance, in FIG. 2B, the first and second pixels 1 and 2 have increased capacitances along with the third and fourth pixels 3 and 4, to improve the visibility of the third and fourth pixels 3 and 4. However, the visibility of the first and second pixels 1 and 2 may decrease. As a result, it may cause a problem wherein the pixel values sensed by the first and second pixels 1 and 2 decrease (as shown by their respective shaded and semi-shaded areas), as shown in FIG. 2B. To solve this problem, an approach to increasing the gains of the first and second pixels 1 and 2 may be employed. In such an approach, the pixel values of the first and second pixels 1 and 2 in FIG. 2B increases. However, noise also increases accordingly.